Various methods and apparatus for the determination of the concentrations of medically significant components of body fluids are known. There are, for example, the methods and apparatus illustrated and described in the following listed references: U.S. Pat. Nos. 3,770,607; 3,838,033; 3,902,970; 3,925,183; 3,937,615; 4,005,002; 4,040,908; 4,086,631; 4,123,701; 4,127,448; 4,214,968; 4,217,196; 4,224,125; 4,225,410; 4,230,537; 4,260,680; 4,263,343; 4,265,250; 4,273,134; 4,301,412; 4,303,887; 4,366,033; 4,407,959; 4,413,628; 4,420,564; 4,431,004; 4,436,094; 4,440,175; 4,477,314; 4,477,575; 4,499,423; 4,517,291; 4,654,197; 4,671,288; 4,679,562; 4,682,602; 4,703,756; 4,711,245; 4,734,184; 4,750,496; 4,759,828; 4,789,804; 4,795,542; 4,805,624; 4,816,224; 4,820,399; 4,871,258; 4,897,162; 4,897,173; 4,919,770; 4,927,516; 4,935,106; 4,938,860; 4,940,945; 4,970,145; 4,975,647; 4,999,582; 4,999,632; 5,053,199; 5,011,290; 5,108,564; 5,128,015; 5,160,980; 5,232,668; 5,243,516; 5,246,858; 5,269,891; 5,284,770; 5,288,636; 5,312,762; 5,352,351; 5,366,609; 5,371,687; 5,379,214; 5,385,846; 5,395,504; 5,469,846; 5,508,171; 5,508,203; 5,509,410; 5,512,489; 5,522,255; 5,594,906; 5,686,659; 5,710,622; 5,789,664; 5,792,944; 5,832,921; 5,841,023; 5,942,102; and 5,997,817: WO98/35225; WO99/28736; and WO99/32881 and certain references cited in WO99/32881: German Patent Specification 3,228,542: European Patent Specifications: 206,218; 230,472; 241,309; 255,291; and, 471,986: and, Japanese Published Patent Applications JP 63-128,252 and 63-111,453.
There are also the methods and apparatus described in: Talbott, et al, “A New Microchemical Approach to Amperometric Analysis,” Microchemical Journal, Vol. 37, pp. 5-12 (1988); Morris, et al, “An Electrochemical Capillary Fill Device for the Analysis of Glucose Incorporating Glucose Oxidase and Ruthenium (III) Hexamine as Mediator, Electroanalysis,” Vol. 4, pp. 1-9 (1992); Cass, et al, “Ferrocene-Mediated Enzyme Electrode for Amperometric Determination of Glucose,” Anal. Chem., Vol. 56, pp. 667-671 (1984); Zhao, “Contributions of Suspending Medium to Electrical Impedance of Blood,” Biochimica et Biophysica Acta, Vol. 1201, pp. 179-185 (1994); Zhao, “Electrical Impedance and Haematocrit of Human Blood with Various Anticoagulants,” Physiol. Meas., Vol. 14, pp. 299-307 (1993); Muller, et al., “Influence of Hematocrit and Platelet Count on Impedance and Reactivity of Whole Blood for Electrical Aggregometry,” Journal of Pharmacological and Toxicological Methods, Vol. 34, pp. 17-22 (1995); Preidel, et al, “In Vitro Measurements with Electrocatalytic Glucose Sensor in Blood,” Biomed. Biochim. Acta, Vol. 48, pp. 897-903 (1989); Preidel, et al, “Glucose Measurements by Electrocatalytic Sensor in the Extracorporeal Blood Circulation of a Sheep,” Sensors and Actuators B, Vol. 2, pp. 257-263 (1990); Saeger, et al, “Influence of Urea on the Glucose Measurement by Electrocatalytic Sensor in the Extracorporeal Blood Circulation of a Sheep,” Biomed. Biochim. Acta, Vol. 50, pp. 885-891 (1991); Kasapbasioglu, et al, “An Impedance Based Ultra-Thin Platinum Island Film Glucose Sensor,” Sensors and Actuators B, Vol. 13-14, pp. 749-751 (1993); Beyer, et al, “Development and Application of a New Enzyme Sensor Type Based on the EIS-Capacitance Structure for Bioprocess Control,” Biosensors & Bioelectronics, Vol. 9, pp. 17-21 (1994); Mohri, et al, “Characteristic Response of Electrochemical Nonlinearity to Taste Compounds with a Gold Electrode Modified with 4-Aminobenzenethiol,” Bull. Chem. Soc. Jpn., Vol. 66, pp. 1328-1332 (1993); Cardosi, et al, “The Realization of Electron Transfer from Biological Molecules to Electrodes,” Biosensors Fundamentals and Applications, chapt. 15 (Turner, et al, eds., Oxford University Press, 1987); Mell, et al, “Amperometric Response Enhancement of the Immobilized Glucose Oxidase Enzyme Electrode,” Analytical Chemistry, Vol. 48, pp. 1597-1601 (September 1976); Mell, et al, “A Model for the Amperometric Enzyme Electrode Obtained Through Digital Simulation and Applied to the Immobilized Glucose Oxidase System,” Analytical Chemistry, Vol. 47, pp. 299-307 (February 1975); Myland, et al, “Membrane-Covered Oxygen Sensors: An Exact Treatment of the Switch-on Transient,” Journal of the Electrochemical Society, Vol. 131, pp. 1815-1823 (August 1984); Bradley, et al, “Kinetic Analysis of Enzyme Electrode Response,” Anal. Chem., Vol. 56, pp. 664-667 (1984); Koichi, “Measurements of Current-Potential Curves, 6, Cottrell Equation and its Analogs. What Can We Know from Chronoamperometry?” Denki Kagaku oyobi Kogyo Butsuri Kagaku, Vol. 54, no. 6, pp. 471-5 (1986); Williams, et al, “Electrochemical-Enzymatic Analysis of Blood Glucose and Lactate,” Analytical Chemistry, Vol. 42, no. 1, pp. 118-121 (January 1970); and, Gebhardt, et al, “Electrocatalytic Glucose Sensor,” Siemens Forsch.-u. Entwickl.-Ber. Bd., Vol. 12, pp. 91-95 (1983).
The disclosures of these references are hereby incorporated herein by reference. This listing is not intended as a representation that a complete search of all relevant prior art has been conducted, or that no better references than those listed exist. Nor should any such representation be inferred.
The clinically significant index of blood glucose, as used in the diagnosis and management of diabetes, is its concentration in the serum, the clear fraction of whole blood after separation of red cells, white cells and plasma. The concentration is determined by any of a variety of chemical and electrochemical methods.
Typically, a predefined sample of serum is transferred from the blood collection receptacle to a reaction vessel in which reagents are combined to produce a chemical reaction whose product is proportional to the total amount of glucose contained in the sample. The product of the chemical reaction can be quantified by gauging optical or electrochemical changes in the reaction mixture, represented as electrical signals or numerical values in digital format. Internal measurement units so obtained, such as optical absorbency, microamperes, and so on, can be converted into reportable clinical units such as milligrams per deciliter or the like, by a blood glucose testing instrument calibration process performed as part of the overall clinical procedure. Internal instrument responses are measured on a series of serum-based reference standards, and the responses and corresponding concentration values are plotted graphically to produce a calibration curve, or fitted computationally to a mathematical function representing concentration in terms of an instrument's response. Advances in computing since the development of many of the known clinical methods have permitted performance of complex data processing and calculation functions directly in even the smallest hand-held instruments.
In a serum-based assay system, the translated glucose concentration values are reported directly in units desired by the clinician. The excluded blood fractions do not figure either in the measurement or the translation process. This is not the case for a system in which the sample is in the form of whole blood. The coexisting blood fractions may affect the reported values, either through physical or chemical interference with the measurement process per se or due to their physical displacement of serum in the sample volume. That is, because the wet chemistry measures total glucose in the sample, the reported serum concentrations will vary as the relative fraction of the serum component varies from sample to sample (usually from individual to individual). In practice, the principle interference is from the red and white blood cells.
For these and a variety of other reasons related to the manner in which blood glucose monitors and test strips are distributed, the calibration procedure for monitors is relatively complicated. Typically, calibration is performed in two steps. In the first step, a large number of test strips in multiple whole blood monitors measure a series of synthetic working standards having a range of predetermined concentrations. The standards are quite stable, and are available in relatively large quantities. The standards' values are used to construct (a) working relationship(s) between the monitors' responses and the standards' concentrations. This step provides a large number of determinations which are then pooled statistically for increased precision over the monitor and test strip populations.
Then, in the second step, instrument response measurements are performed on whole blood samples from a relatively smaller population of human donors, which are paired with glucose values determined by the reference serum methodology to adjust the working curve to yield true reportable values. This step accounts on a statistical basis for the unique properties of human blood. Thus, human donors provide, as they should, the ultimate basis for monitor results.
The decimal fraction of whole blood volume occupied by red cells is known as hematocrit. Hematocrit correction on whole blood determinations would not be necessary if hematocrit did not vary from person to person, because its effect could be taken into account in the calibration process by introducing whole blood samples into the instrument during calibration and relating their internally-measured numbers to corresponding serum values determined during the reference phase of the calibration process. However, hematocrit values can vary between about 0.2 for individuals who suffer from anemia and about 0.6 for newborns. Since a glucose determination is subject to about a one percent variation for each percent hematocrit variation, the resulting uncertainty in indicated glucose concentration would be clinically undesirable. Therefore, unless a glucose determination methodology is employed which is hematocrit-independent, for closest agreement with laboratory reference methods, individual determinations of glucose concentration must be compensated or corrected for hematocrit. This applies both to the donors whose blood is used during the calibration process and to the patients whose glucose concentrations are of interest to the clinician.